We aimed to investigate the effects of meteorological factors and air pollutants on out-of-hospital cardiac arrest (OHCA) according to seasonal variations because the roles of these factors remain controversial to date.A total of 38 928 OHCAs of cardiac origin that occurred within eight metropolitan areas between 2012 and 2016 were identified from the Korean nationwide emergency medical service database. A time series multilevel approach based on Poisson analysis following a Granger causality test was used to analyse the influence of air pollution and 13 meteorological variables on OHCA occurrence.Particulate matter (PM) ≤2.5 µm (PM2.5), average temperature, daily temperature range and humidity were significantly associated with a higher daily OHCA risk (PM2.5: 1.59%; 95% CI: 1.51% to 1.66% per 10µg/m3, average temperature 0.73%, 95% CI: 0.63% to 0.84% per 1°C, daily temperature range: 1.05%, 95% CI: 0.63% to 1.48% per 1°C, humidity -0.48, 95% CI: -0.40 to -0.56 per 1%) on lag day 1. In terms of the impact of these four risk factors in different seasons, average temperature and daily temperature range were highly associated with OHCA in the summer and winter, respectively. However, only PM2.5 elevation (to varying extents) was an independent and consistent OHCA risk factor irrespective of the season.PM2.5, average temperature, daily temperature range and humidity were independently associated with OHCA occurrence in a season-dependent manner. Importantly, PM2.5 was the only independent risk factor for OHCA occurrence irrespective of seasonal changes.